Several processes in the petrochemical refining industry require information on moisture levels of component streams to optimize the performance of the process or to prevent deterioration of, or damage to, some part of the system. For example, the performance characteristics of chromatographic separation of organic components by adsorbents are affected by the water content of component streams and under dynamic equilibrium conditions there often is a narrow regime of water content within which separation is maximized. The performance of the adsorbents themselves may significantly deteriorate upon water adsorption, or the adsorbents may physically deteriorate at high water levels. Since the processes in question, such as chromatographic separations, often are continuous with the nature of the feed stream time variable, not only is a reliable method of determining water level required, but the method may need to be suitable for frequent periodic use. Although continuous monitoring of water content may be excessive, the monitoring frequency may be relatively high, thereby requiring a short measurement time cycle. Additionally, obtaining individual samples for laboratory analysis is time consuming, inconvenient, and rife with the opportunity of introducing sampling error; methods incorporating on-line measurements are preferable.
Existing water monitors are primarily designed to operate in a gas phase, although two commercially available types are applicable to liquid streams. One type uses a hydrophilic material, usually alumina (although silica or organic polyimides also have been used), placed between two electrodes. As the water level in a liquid stream changes, the hydrophilic material absorbs or loses water, and its impedance change is monitored by means of an AC signal applied across the electrodes. These sensors have three significant disadvantages: the alumina undergoes a shape change when wet causing a permanent offset of the detector when dried arising from the change in the geometry of the material between the electrodes; the sensors are very slow to respond to changes in water composition, especially when the level is low or decreasing; and the sensors do not measure the water concentration directly but instead depend upon the distribution coefficient of water between the organic phase and the hydrophilic layer. As a consequence of the latter, such sensors are very sensitive to any factors which alter the distribution coefficient, such as temperature and, of even greater relevance, the composition of the organic phase. This dependence of the impedance change on the composition of the organic phase makes the measurement of water content only a relative one rather than an absolute measurement. That is, a calibration curve needs to be established for each organic phase whose water level is being determined, an obviously great inconvenience and significant burden.
The second type of commercially available sensor for water in hydrocarbons is based on infrared (IR) absorption by water. These systems generally use the near IR region of the spectrum (1.8-1.9 microns) although some systems are available which use the mid-range (approximately 2.7 microns). In either case, the sensor simply measures the attenuation of IR radiation through the sample of liquid and may also measure a reference attenuation at a wavelength outside the region where water absorbs. This approach also is extremely sensitive to changes in the organic phase since each component of the latter also attenuates the radiation in a characteristic manner. In our studies errors of 200% and more have been observed using the commercial measuring system when the water level was kept constant and only the organic phase was changed.
Our requirements for a suitable measurement method for the detection of water are manifold. One requirement is that the method be insensitive to changes in the organic liquid phase, especially where the latter is a hydrocarbon. The method needs to be capable of determining water level in liquid hydrocarbons at levels as low as about 10 ppm and at least as high as about 1000 ppm. The method is required to be accurate over this range to within 1% or 5 ppm, whichever is greater, and must be reproducible as well. The method needs to be selective for water, especially as to components such as alcohols, amines, and thiols which might be present in the liquid hydrocarbon medium. Finally, the method needs to be adaptable to near-continuous on-line monitoring of process streams.
Several approaches initially were evaluated and discarded because of severe limitations which became apparent in their evaluation, including methods based on microwave absorption, electrochemical measurements, and calorimetric determinations. A modification of the IR approach was considered where the amount of water would be determined by measuring the change in attenuation of IR radiation by a sample compared with the attenuation of the same sample after drying. It was subsequently noted that the absorption spectrum for water in organic liquids consisted of two peaks corresponding to the symmetric and asymmetric OH stretches. More importantly, it was also found that the total area under the two peaks at a given water concentration varied with the nature of the organic phase. However, a chance observation led to the development of the method which is our invention and which satisfies all the stated criteria. In particular, we noticed that as to the water absorption bands in the infrared spectrum of a water-containing homogeneous organic liquid it is only the OH asymmetric stretching frequency which is variable with the organic component, whereas the symmetric stretching frequency is essentially independent of the organic phase, especially when the latter is a liquid hydrocarbon. This observation is the keystone of our invention as described more fully within, and is somewhat contrary to the report of N. M. Alekseeva and E. E. Yudovich, Zhurnal Prikladnoi Spektroskopii, 15, No. 6, 1076-9 (1971), which indicates that the ratio of intensities of the symmetric and antisymmetric stretching bands remain constant with changes in solvent. The authors recommend use of the antisymmetric stretching band in water measurements in contradistinction to the method used in this invention.
Although one aspect of our invention is a method of measuring water concentration in a liquid hydrocarbon where the method is essentially independent of the hydrocarbon, in another aspect our invention is an online instrument to measure water in hydrocarbons in an automated, maintenance-free manner. The relative invariance of the symmetric stretching frequency of the water molecule provides the key to successfully effecting our invention.